Modern ships can be designed like floating power plants. Such ships can use the electricity generated onboard to provide power for a wide range of applications. Lighting, heating, cooling, ventilation, pumps, navigation systems and cargo related activities are examples of such applications.
Many large cruise ships commonly use diesel electric propulsion systems. One reason for this is the easiness of manoeuvring, especially during docking. An example is to have the propulsion provided by means of several diesel engines coupled to main generators, which drive the electric motor that runs the propeller on the ship. The same generators that are used for propulsion can also be used to generate auxiliary power onboard the vessel for lights, refrigeration, etc.
At sea, power generation onboard can be provided by a main generator that is coupled with one or more electric propulsion engines, in combination with auxiliary generators for generating sufficient power. At berth, the main engines can be shutdown and the auxiliary generators take control of all power generation onboard. To be able to also shut down the auxiliary generators, an external power supply is required while mooring at a quay.
For a modern cruise ship, the power needed while at sea can be up to 18 MW. To be able to handle such an extent of power, high voltage, e.g. 6-11 kV, is used onboard. When the vessel docks at the berth there is no need to produce the same quantity of power to drive the propulsion motors, so therefore a majority of the main generators are shut down and only a few generators are used to manage the power needed, e.g. for powering hotel equipment.
Shore side power supply has been used for supplying ferries with electricity. Since many ferries are always docked in the same position at the same quay it easy to provide a suitable connection. Today, some other types of commercial ships, such as, cruise, container, and Ro/Ro (roll-on/roll-off) vessels are connected to the electrical grid in ports around the world.
Typical power requirements for Ro/Ro- and Container vessels are high voltage, like 6.6 kV and 7.5 MVA power. Cruise vessels can also require high voltage, e.g. 6.6 kV or 11 kV, depending on the power demand of the vessel.
In 2000 a high voltage ship connection for vessel power supply was made in the port of Gothenburg.
A problem with power supplies to ships is that different countries use AC power transmissions at different frequencies, e.g. 60 Hz in U.S.A. and 50 Hz in European countries. Each ship usually uses either 50 or 60 Hz systems for their equipment. Hence, when power is supplied from a port, this causes a problem where the frequency of the port and the ship differs. Frequency converters can then be used to convert the frequency from the grid frequency of the port to the frequency of the ship.
However, frequency conversion needs to be done with great efficiency to reduce cost and increase reliability.